1. Field of the Invention
This invention relates generally to catheter-type devices used for various purposes including for example in the practice of angiography and angioplasty techniques. More specifically, the invention pertains to catheters and a method for producing catheters having fuseless and seamless structures that exhibit selectively varied physical characteristics along their lengths.
2. Brief Description of the Prior Art
Catheters used within a vascular system are required to have a number of apparently conflicting physical characteristics. For example, the catheter must be sufficiently rigid in the proximal region to enable its distal end to be maneuvered by manipulation of its proximal end. Both torsional as well as axial forces must therefore be transferable along the catheter's entire length despite the substantial frictional resistance that may be encountered. It is however simultaneously necessary for the catheter's distal end to be sufficiently soft so as not to traumatize the vascular walls when being advanced and sufficiently flexible to enable it to readily follow a potentially tortuous vascular path. Sufficient flexibility and an atraumatic tip are of similar importance in over-the-wire type applications wherein the catheter's leading edge may scrape against vessel walls as it is being advanced along a guide wire positioned in those vessels and wherein excessive rigidity would prevent the catheter from following a tightly curved wire. The catheter may further be limited with respect to its overall diameter in order to permit its introduction into small vessels yet may be called upon to handle substantial flow rates and pressures. In addition to variation in mechanical properties, certain applications may call for variation in the catheter's thermal, electrical, chemical, optical, and permeability properties at certain locations along its length. In all cases the catheter material must be non-toxic and should be non-thrombogenic, smooth-walled and resistant to kinking.
The design requirements set forth above have traditionally been approached with the use of multi-piece structures wherein for example a rather soft and flexible catheter tip is fused onto or otherwise attached to a relatively rigid, and high tensile strength catheter extrusion. Such designs are plagued by a number of inherent disadvantages including for example, the potentially compromised strength of the catheter at the junction between the tip and the catheter body which could result in a failure at that location. Additionally, since catheters are disposed of after a single use and cost is consequently of major concern, the production of at least two separate components and their required assembly can be economically prohibitive.
An alternative approach is exemplified by the catheter structure described in U.S. Pat. No. 4,385,635 to Ruiz. The required flexibility near the tip and the necessary rigidity in the rest of the catheter is achieved by a structure wherein a soft elastomeric sleeve encloses an inner reinforcing tube which tapers to zero wall thickness just short of the sleeve's end. The unreinforced sleeve end serves as the soft catheter tip while the necessary rigidity is imparted to the rest of the catheter by the presence of the reinforcing tube therein. Disadvantages associated with such a design include the substantial cost of manufacture and the fact that only a limited number of parameters would appear to be variable in this manner.
Weldon in U.S. Pat. No. 4,963,306 proposes a fuseless catheter structure wherein the desired variation of certain physical properties is achieved within a single extrusion. Partially polymerized material is extruded to form the catheter after which a selected portion thereof is induced to undergo solid state polymerization. Those portions that have undergone only a limited degree of polymerization remain flexible, while the further polymerization of other portions increases their strength and rigidity. A fuseless catheter appropriately treated retains a soft, flexible, and atraumatic tip while the remaining portion is rendered stiffer and stronger and is capable of withstanding higher internal pressures. It is stated that such catheter can be maneuvered through a circuitous vascular path without subjecting the vessel walls to trauma while radiopaque fluid can be delivered at higher flow rates with minimized danger of rupture. Weldon achieves solid state polymerization at selected portions of the catheter by subjecting those portions to a temperature above the boiling temperature of water and below the melting point of the extrudate while maintaining the balance of the catheter at substantially lower temperatures. It is conjectured that the removal of water from within the polymer network permits more complete polymerization to occur.
Disadvantages associated with the use of solid state polymerization techniques include the relatively modest enhancement of physical properties that is realizable, the relatively slow rate at which the cold polymerization reactions proceed, the relative difficulty involved in controlling the degree of polymerization achieved due to the exothermic nature of the reaction and although stiffness is increased, kink resistance, defined in terms of bend radius, is diminished.